Only one account can control sub-accounts. Sub-accounts can not have multiple 'owners'.
So, Main reseller > Sub reseller > User. Both reseller's can control the user if you use the shadowing feature. But, the main reseller has to shadow the sub reseller account and in turn also control the user account. Not the ideal way since the main reseller can fully access the sub reseller account. That would be the only way.
So, Main reseller > Sub reseller > User. Both reseller's can control the user if you use the shadowing feature. But, the main reseller has to shadow the sub reseller account and in turn also control the user account. Not the ideal way since the main reseller can fully access the sub reseller account. That would be the only way.